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1Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
DC Choppers
2Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Introduction• Chopper is a static device.
• A variable dc voltage is obtained from a constant dc voltage source.
• Also known as dc-to-dc converter.
• Widely used for motor control.
• Also used in regenerative braking.
• Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control.
3Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Choppers are of Two Types
Step-down choppers. Step-up choppers.
In step down chopper output voltage is less than input voltage.
In step up chopper output voltage is more than input voltage.
4Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Principle Of Step-down Chopper
V
i0
V 0
C hopper
R
+
5Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• A step-down chopper with resistive load.
• The thyristor in the circuit acts as a switch.
• When thyristor is ON, supply voltage appears across the load
• When thyristor is OFF, the voltage across the load will be zero.
6Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V d c
v 0
V
V /R
i0
Id c
t
t
tO N
T
tO F F
7Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
verage value of output or load voltage.
verage value of output or load current.
Time interval for which SCR conducts.
Time interval for which SCR is OFF.
Period of switching
dc
dc
ON
OFF
ON OFF
V A
I A
t
t
T t t
or chopping period.
1 Freq. of chopper switching or chopping freq.f
T
8Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Average Output Voltage
.
duty cycle
ONdc
ON OFF
ONdc
ON
tV V
t t
tV V V d
T
tbut d
t
9Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
2
0
Average Output Current
RMS value of output voltage
1 ON
dcdc
ONdc
t
O o
VI
RtV V
I dR T R
V v dtT
10Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
2
0
2
But during ,
Therefore RMS output voltage
1
.
.
ON
ON o
t
O
ONO ON
O
t v V
V V dtT
tVV t V
T T
V d V
11Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
2
2
Output power
But
Output power
O O O
OO
OO
O
P V I
VI
R
VP
R
dVP
R
12Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Effective input resistance of chopper
The output voltage can be varied by
varying the duty cycle.
idc
i
VR
I
RR
d
3.1 Basic DC to DC converters 3.1.1Buck converterSPDT switch changes dc
component
Switch output voltage waveform
Duty cycle D: 0 ≤ D ≤ 1
complement D: D´ = 1 - D
+ -
+
-
V(t)
R
Vg
+
-
Vs(t)
Vs(t) Vg
switch position:
DTs
D Ts
0
1
1
2
t
1
2
0
DTs
Ts
Dc component of switch output voltage
Vs(t) Vg
<Vs>=DVg
DTs
Ts
0
t
Fourier analysis:DC component =average value:
0
area= D Ts Vg
<Vs>
=
∫
0
Ts Vs(t)
dt
Ts
1
<Vs>
=
=DVg
1
Ts DTsVg
Insertion of low- pass filter to remove switching harmonics and pass only dc component
+ -
+
-
V(t)
R
Vg
+
-
Vs(t)
1
2
L
C
v≈<vs>
=DVg
V
Vg
o
0 1 D
• Basic operation principle of buck converter
+ -
+
-
V(t)
R
Vg
+
-
Vs(t)
1
2
L
C
Buck converter with
ideal switch
Realization using
power MOSFET
and diode
+ -
+
-
VL(t)
ic(t)
Vg
iL(t)
t
DTs
Ts
+
L
D1
R
Thought process in analyzing basic DC/DC converters
1) Basic operation principle (qualitative analysis)
–How does current flows during different switching states
–How is energy transferred during different switching states
2) Verification of small ripple approximation
3) Derivation of inductor voltage waveform during different switching states
4) Quantitative analysis according to inductor volt-second balance or capacitor charge balance
• Actual output voltage waveform of buck converter
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
1
2
L
C
Buck converter
containing practical
low-pass filter
ic(t)
iL(t)
Actual output voltage
waveform
v(t ) = V + v ripple(t)
v(t )
V
0
t
Actual waveform
v(t ) = V + v ripple(t)
DC component V
• Buck converter analysis: inductor current waveform
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
1
2
L
C
original
converter
ic(t)
iL(t)
Switch in position 1
Switch in position 2
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
L
C
ic(t)
iL(t)
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
L
C
ic(t)
iL(t)
• Inductor voltage and current subinterval 1: switch in position 1
Inductor voltage
vL=Vg - v(t) Small ripple approximation:
vL=Vg - V
Knowing the inductor voltage, we can now find the inductor current via
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
L
C
ic(t)
iL(t)
vL(t)=L diL(t)
dt Solve for the slope:
diL(t)
dt =
vL(t)
L ≈ Vg - V
L
the inductor current changes with an
essentially constant slope
• Inductor voltage and current subinterval 2: switch in position 2
Inductor voltage
vL= - v(t) Small ripple approximation:
vL ≈ - V
Knowing the inductor voltage, we can now find the inductor current via
vL(t)=L diL (t)
dt Solve for the slope:
diL(t)
dt ≈
V L
the inductor current changes with an
essentially constant slope -
+ -
+
-
V(t)
R
Vg
+
-
VL(t)
L
C
ic(t)
iL(t)
Inductor voltage and current waveforms
VL(t)
Vg -V
switch position:
DTs
DTs
-V
1
1
2
t
vL(t)=L diL (t)
dt iL(t)
t
DTs
Ts
0
I
iL(0)
iL(DTs)
Vg -V
L
-V
L
Δ iL
Determination of inductor current ripple magnitude
(changes in iL)=(slope)(length of subinterval)
Vg -V
L
2Δ iL
DTs
=
Δ iL
=
Vg -V
2L DTs
L
=
Vg -V
2Δ iL
DTs
iL(t)
DTs
Ts
0
I
iL(0)
iL(DTs)
Vg -V
L
-V
L
Δ iL
Inductor current waveform during start-up transient
iL(t)
t
DTs
Ts
0
When the converter operates in equilibrium:
iL(0)=0
iL(Ts)
Vg –v(t)
L
-v(t)
L
2Ts
iL(nTs)
nTs
(n+1)Ts
iL((n+1)Ts)
iL((n+1)Ts)= iL(nTs)
• The principle of inductor volt- second balance: DerivationInductor defining relation:
Integrate over one complete switching period:
In periodic steady state, the net changes in inductor current is zero:
Hence, the total area(or volt-seconds)under the inductor voltage waveform is zero whenever the converter operates in steady state.
An equivalent form:
The average inductor voltage is zero in steady state.
vL(t)=L diL (t)
dt
∫ 0
Ts VL(t)
dt
L 1
iL(Ts) -
iL(0)=
∫ 0
Ts VL(t)
dt
=
0
<vL>
=
∫ 0
Ts VL(t)
dt
Ts 1
=
0
• Inductor volt-second balance:Buck converter example
Integral of voltage waveform is area of rectangles:
average voltage is
Equate to zero and solve for V:
inductor voltage waveform
previously derived:
VL(t)
Vg -V
DTs
-V
t
total area λ
∫ 0
Ts VL(t)
dt
=
λ
= (Vg –V)( DTs)+( -V) ( DTs)
<vL>
=
Ts λ
=D (Vg –V) +D'( -V)
0=D Vg –(D+D')V= D Vg –V
V=D Vg
3.1.2Boost converter
• Boost converter example
+ -
+
-
v
R
Vg
+
-
vL(t)
1
2
L
C
Boost converter
with ideal switch
iL(t)
iC(t)
Realization using
power MOSFET
and diode
+ -
ic(t)
Vg
iL(t)
t
DTs
Ts
+
-
VL(t)
L
D1
R
+ -
Q1
+
-
v
C
• Boost converter analysis
original
converter
Switch in position 1
Switch in position 2
+ -
+
-
v
R
Vg
+
-
vL(t)
1
2
L
C
iL(t)
+ -
+
-
v
R
Vg
+
-
vL(t)
L
C
iL(t)
iC(t)
iC(t)
+ -
+
-
v
R
Vg
+
-
vL(t)
L
C
iL(t)
iC(t)
• Subinterval 1: switch in position 1
Inductor voltage and capacitor current
vL=Vg
Small ripple approximation:
iC= - v/R + -
+
-
v
R
Vg
+
-
vL(t)
L
C
iL(t)
iC(t)
vL=Vg
iC= - V/R
• Subinterval 2: switch in position 2
Inductor voltage and capacitor current
vL=Vg -v
Small ripple approximation:
iC=iL - v/R
vL=Vg -V
iC= I - V/R
+ -
+
-
v
R
Vg
+
-
vL(t)
L
C
iL(t)
iC(t)
• Inductor voltage and capacitor current waveforms
VL(t)
Vg
DTs
D'Ts
Vg -V
t
iC(t)
-V/R
DTs
D'Ts
1 –V/R
t
• Inductor volt- second balance
VL(t)
Vg
DTs
D'Ts
Vg -V
t
∫
0
Ts VL(t)
dt
= ( Vg) DTs+(Vg –V) D'Ts
Net volt-seconds applied to inductor
over one switching period:
Equate to zero and collect terms:
Vg(D+ D')-VD'=0
Solve for V:
V=
Vg
D'
The voltage conversion ratio is therefore:
V
Vg
D'
M(D)=
=
=
1
1-D
1
Conversion ratio M(D) of the boost converter
D'
M(D)=
=
1
1-D
1
D
M(D)
0
0
1
2
3
4
5
0.2
0.4
0.6
0.8
1
• Determination of inductor current dc component
Vg/R
I
D
0
0
2
4
6
8
0.2
0.4
0.6
0.8
1
iC(t)
-V/R
DTs
D'Ts
I –V/R
t
Capacitor charge balance:
∫
0
Ts iC(t)
dt
=(-
)D'Ts
V
R
)DTs
+(I-
V
R
Collect terms and equate to zero:
-
V
R
(D+D')+I D'=0
Solve for I:
V
D'R
I=
Eliminate V to express in terms of Vg:
Vg
D' I=
2
R
Continuous- Conduction- Mode (CCM) and Discontinuous Conduction-Mode (DCM) of boost
M E
VD L
V uo EM
a)
36Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Methods Of Control
• The output dc voltage can be varied by the following methods.
– Pulse width modulation control or constant frequency operation.
– Variable frequency control.
37Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Pulse Width Modulation
• tON is varied keeping chopping frequency ‘f’ & chopping period ‘T’ constant.
• Output voltage is varied by varying the ON time tON
38Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V 0
V
V
V 0
t
ttO N
tO N tO F F
tO F F
T
39Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Variable Frequency Control
• Chopping frequency ‘f’ is varied keeping either tON or tOFF constant.
• To obtain full output voltage range, frequency has to be varied over a wide range.
• This method produces harmonics in the output and for large tOFF load current may become discontinuous
40Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
v 0
V
V
v 0
t
t
tO N
tO N
T
T
tO F F
tO F F
41Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Step-down ChopperWith R-L Load
V
i0
V 0
C hopper
R
LFW D
E
+
42Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When chopper is ON, supply is connected across load.
• Current flows from supply to load.
• When chopper is OFF, load current continues to flow in the same direction through FWD due to energy stored in inductor ‘L’.
43Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Load current can be continuous or discontinuous depending on the values of ‘L’ and duty cycle ‘d’
• For a continuous current operation, load current varies between two limits Imax and Imin
• When current becomes equal to Imax the chopper is turned-off and it is turned-on when current reduces to Imin.
44Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utpu tvo ltage
O utpu tcurrent
v 0
V
i0
Im a x
Im in
t
t
tO N
T
tO F F
C ontinuouscurrent
O utpu tcurrent
t
D iscon tinuouscurrent
i0
45Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expressions For Load Current
iO For Continuous Current Operation When
Chopper Is ON (0 t tON)
46Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
V
i0
V 0
R
L
E
+
-
47Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
min
Taking Laplace Transform
. 0
At 0, initial current 0
OO
O O O
O
O
diV i R L E
dt
V ERI S L S I S i
S S
t i I
IV EI S
RR SLS SLL
48Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
Taking Inverse Laplace Transform
1
This expression is valid for 0 ,
i.e., during the period chopper is ON.
At the instant the chopper is turned off,
load c
R Rt t
L LO
ON
V Ei t e I e
R
t t
maxurrent is O ONi t I
49Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
When Chopper is OFF
i0
R
L
E
50Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
When Chopper is OFF 0
0
Talking Laplace transform
0 0
Redefining time origin we have at 0,
initial current 0
OFF
OO
O O O
O
t t
diRi L E
dt
ERI S L SI S i
St
i I
51Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
max
Taking Inverse Laplace Transform
1
O
R Rt t
L LO
I EI S
R RS LS SL L
Ei t I e e
R
52Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
The expression is valid for 0 ,
i.e., during the period chopper is OFF
At the instant the chopper is turned ON or at
the end of the off period, the load current is
OFF
O OFF
t t
i t I
53Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
max
max
max min
min
From equation
1
At ,
To Find &
1
R Rt t
L LO
ON O
dRT dRT
L L
V Ei t e I e
R
t t dT i t I
V EI e I e
I I
R
54Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
min
From equation
1
At ,
1
R Rt t
L LO
OFF ON O
OFF
Ei t I e e
R
t t T t i t I
t t d T
55Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
1 1
min max
min
max min
max
1
Substituting for in equation
1
we get,
1
1
d RT d RT
L L
dRT dRT
L L
dRT
L
RT
L
EI I e e
R
I
V EI e I e
R
V e EI
R Re
56Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
1 1
min max
min
max min
Substituting for in equation
1
we get,
1
1
is known as the steady state ripple.
d RT d RT
L L
dRT
L
RT
L
I
EI I e e
R
V e EI
R Re
I I
57Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max min
max min
Therefore peak-to-peak ripple current
Average output voltage
.
Average output current
2
dc
dc approx
I I I
V d V
I II
58Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min max
min
max minmin
Assuming load current varies linearly
from to instantaneous
load current is given by
. 0O ON
O
I I
I ti I for t t dT
dTI I
i I tdT
59Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
20
0
2
max minmin
0
2min max min2 2max min
min
0
RMS value of load current
1
1
21
dT
O RMS
dT
O RMS
dT
O RMS
I i dtdT
I I tI I dt
dT dT
I I I tI II I t dt
dT dT dT
60Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
12 2
max min2min min max min
20
0
2
max minmin
0
RMS value of output current
3
RMS chopper current
1
1
O RMS
dT
CH
dT
CH
I II I I I I
I i dtT
I II I t dt
T dT
61Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
12 2
max min2min min max min3
Effective input resistance is
CH
CH O RMS
iS
I II d I I I I
I d I
VR
I
62Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Where
Average source currentS
S dc
idc
I
I dI
VR
dI
63Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Principle Of Step-up Chopper
+
V OV
C hopper
CLOAD
DLI
+
64Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Step-up chopper is used to obtain a load voltage higher than the input voltage V.
• The values of L and C are chosen depending upon the requirement of output voltage and current.
• When the chopper is ON, the inductor L is connected across the supply.
• The inductor current ‘I’ rises and the inductor stores energy during the ON time of the chopper, tON.
65Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, tOFF.
• The current tends to decrease resulting in reversing the polarity of induced EMF in L.
• Therefore voltage across load is given by
. ., O O
dIV V L i e V V
dt
66Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• A large capacitor ‘C’ connected across the load, will provide a continuous output voltage .
• Diode D prevents any current flow from capacitor to the source.
• Step up choppers are used for regenerative braking of dc motors.
67Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expression For Output Voltage
Assume the average inductor current to be
during ON and OFF time of Chopper.
Voltage across inductor
Therefore energy stored in inductor
= . .
Where
When Chopper
period of chopper.
is ON
ON
ON
I
L V
V I t
t ON
68Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
(energy is supplied by inductor to load)
Voltage across
Energy supplied by inductor
where period of Chopper.
Neg
When Chopper
lecting losses, energy stored in inductor
is OFF
O
O OFF
OFF
L V V
L V V It
t OFF
L
= energy supplied by inductor L
69Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Where
T = Chopping period or period
of switching.
ON O OFF
ON OFFO
OFF
OON
VIt V V It
V t tV
t
TV V
T t
70Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
1
1
1
1
Where duty cyle
ON OFF
OON
O
ON
T t t
V Vt
T
V Vd
td
T
71Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
For variation of duty cycle ' ' in the
range of 0 1 the output voltage
will vary in the range O
O
d
d V
V V
72Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Performance Parameters• The thyristor requires a certain minimum time to
turn ON and turn OFF.
• Duty cycle d can be varied only between a min. & max. value, limiting the min. and max. value of the output voltage.
• Ripple in the load current depends inversely on the chopping frequency, f.
• To reduce the load ripple current, frequency should be as high as possible.
73Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Classification Of Choppers
• Choppers are classified as – Class A Chopper– Class B Chopper– Class C Chopper– Class D Chopper– Class E Chopper
74Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class A Chopper
V
C hopper
FW D
+
v 0
v 0
i0
i0
LOAD
V
75Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When chopper is ON, supply voltage V is connected across the load.
• When chopper is OFF, vO = 0 and the load current continues to flow in the same direction through the FWD.
• The average values of output voltage and current are always positive.
• Class A Chopper is a first quadrant chopper .
76Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Class A Chopper is a step-down chopper in which power always flows form source to load.
• It is used to control the speed of dc motor.
• The output current equations obtained in step down chopper with R-L load can be used to study the performance of Class A Chopper.
77Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utpu t cu rren t
Thyristo rgate pu lse
O utpu t vo ltage
ig
i0
v 0
t
t
ttO N
T
C H O N
FW D C onduc ts
78Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class B Chopper
V
C hopper
+
v 0
v 0
i0
i0
L
E
R
D
79Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure.
• During the ON period of the chopper, the inductance L stores energy.
• When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply.
80Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Average output voltage is positive.• Average output current is negative. • Therefore Class B Chopper operates in second
quadrant.• In this chopper, power flows from load to
source.• Class B Chopper is used for regenerative
braking of dc motor.• Class B Chopper is a step-up chopper.
81Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
O utpu t cu rren t
D con d u cts C h o pp e r
con d u cts
Thyristo rgate pu lse
O utpu t vo ltage
ig
i0
v 0
t
t
t
Im in
Im ax
T
tO NtO F F
82Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Expression for Output Current
83Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
For the initial condition i.e.,
During the interval diode 'D' conduc
at 0
The solution of the ab
ts
voltage equation
ove equation is obtained
along similar lines as in s
is given by
OO
O
LdiV Ri E
dt
i t I t
tep-down chopper
with R-L load
84Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
min
max
max min
During the interval chopper is ON voltage
equation is g
1 0
At
1
0
iven by
OFF OFF
R Rt t
L LO OFF
OFF O
R Rt t
L L
OO
V Ei t e I e t t
R
t t i t I
V EI e I e
R
LdiRi E
dt
85Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
max
max
min
min max
Redefining the time origin, at 0
The solution for the stated initial condition is
1 0
At
1ON ON
O
R Rt t
L LO ON
ON O
R Rt t
L L
t i t I
Ei t I e e t t
R
t t i t I
EI I e e
R
86Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class C Chopper
V
C hopper
+
v 0
D 1
D 2C H 2
C H 1
v 0i0
i0
L
E
R
87Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Class C Chopper is a combination of Class A and Class B Choppers.
• For first quadrant operation, CH1 is ON or D2 conducts.
• For second quadrant operation, CH2 is ON or D1 conducts.
• When CH1 is ON, the load current is positive.• The output voltage is equal to ‘V’ & the load
receives power from the source. • When CH1 is turned OFF, energy stored in
inductance L forces current to flow through the diode D2 and the output voltage is zero.
88Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Current continues to flow in positive direction.• When CH2 is triggered, the voltage E forces
current to flow in opposite direction through L and CH2 .
• The output voltage is zero.• On turning OFF CH2 , the energy stored in the
inductance drives current through diode D1 and the supply
• Output voltage is V, the input current becomes negative and power flows from load to source.
89Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Average output voltage is positive• Average output current can take both positive
and negative values.• Choppers CH1 & CH2 should not be turned
ON simultaneously as it would result in short circuiting the supply.
• Class C Chopper can be used both for dc motor control and regenerative braking of dc motor.
• Class C Chopper can be used as a step-up or step-down chopper.
90Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseof C H 2
G ate pu lseof C H 1
O utput cu rren t
O utpu t vo ltage
ig 1
ig 2
i0
V 0
t
t
t
t
D 1 D 1D 2 D 2C H 1 C H 2 C H 1 C H 2
O N O N O N O N
91Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class D Chopper
V+ v 0
D 2
D 1 C H 2
C H 1
v 0
i0
L ER i0
92Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Class D is a two quadrant chopper.
• When both CH1 and CH2 are triggered simultaneously, the output voltage vO = V and output current flows through the load.
• When CH1 and CH2 are turned OFF, the load current continues to flow in the same direction through load, D1 and D2 , due to the energy stored in the inductor L.
• Output voltage vO = - V .
93Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Average load voltage is positive if chopper ON time is more than the OFF time
• Average output voltage becomes negative if tON < tOFF .
• Hence the direction of load current is always positive but load voltage can be positive or negative.
94Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseof C H 2
G ate pu lseof C H 1
O utpu t curren t
O utpu t vo ltage
Average v 0
ig 1
ig 2
i0
v 0
V
t
t
t
t
C H ,C HO N1 2 D 1,D 2 Conducting
95Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
G ate pu lseof C H 2
G ate pu lseof C H 1
O utpu t curren t
O utpu t vo ltage
Average v 0
ig 1
ig 2
i0
v 0
V
t
t
t
t
C HC H
1
2
D , D1 2
96Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Class E Chopper
V
v 0
i0L ER
C H 2 C H 4D 2 D 4
D 1 D 3C H 1 C H 3
+
97Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Four Quadrant Operationv 0
i0
C H - C H O NC H - D C onduc ts
1 4
4 2
D D2 3 - C onductsC H - D C onduc ts4 2
C H - C H O NC H - D C onduc ts
3 2
2 4
C H - D C onduc tsD - D C onducts
2 4
1 4
98Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Class E is a four quadrant chopper
• When CH1 and CH4 are triggered, output current iO flows in positive direction through CH1 and CH4, and with output voltage vO = V.
• This gives the first quadrant operation.
• When both CH1 and CH4 are OFF, the energy stored in the inductor L drives iO through D2 and D3 in the same direction, but output voltage vO = -V.
99Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Therefore the chopper operates in the fourth quadrant.
• When CH2 and CH3 are triggered, the load current iO flows in opposite direction & output voltage vO = -V.
• Since both iO and vO are negative, the chopper operates in third quadrant.
100Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• When both CH2 and CH3 are OFF, the load current iO continues to flow in the same direction D1 and D4 and the output voltage vO = V.
• Therefore the chopper operates in second quadrant as vO is positive but iO is negative.
101Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Impulse Commutated Chopper
• Impulse commutated choppers are widely used in high power circuits where load fluctuation is not large.
• This chopper is also known as
– Parallel capacitor turn-off chopper
– Voltage commutated chopper
– Classical chopper.
102Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
LOAD
L
C
IL
L S
V S
+
_
+
_
T 2
T 1
D 1
a
biC
iT 1
v O
+
_
FW D
103Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• To start the circuit, capacitor ‘C’ is initially charged with polarity (with plate ‘a’ positive) by triggering the thyristor T2.
• Capacitor ‘C’ gets charged through VS, C, T2 and load.
• As the charging current decays to zero thyristor T2 will be turned-off.
• With capacitor charged with plate ‘a’ positive the circuit is ready for operation.
• Assume that the load current remains constant during the commutation process.
104Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• For convenience the chopper operation is divided into five modes.– Mode-1
– Mode-2
– Mode-3
– Mode-4
– Mode-5
105Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Mode-1 Operation
LOADL
C
IL
L S
V S
+
_
+
_
T 1
D 1
V C iC
106Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Thyristor T1 is fired at t = 0.
• The supply voltage comes across the load.
• Load current IL flows through T1 and load.
• At the same time capacitor discharges through T1, D1, L1, & ‘C’ and the capacitor reverses its voltage.
• This reverse voltage on capacitor is held constant by diode D1.
107Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Capacitor Discharge Current
sin
1Where
& Capacitor Voltage
cos
C
C
Ci t V t
L
LC
V t V t
108Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Mode-2 Operation
LOAD
C
L S
V S+
_+
_
T 2
V C
IL
IL
109Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Thyristor T2 is now fired to commutate thyristor T1.
• When T2 is ON capacitor voltage reverse biases T1 and turns if off.
• The capacitor discharges through the load from –V to 0.
• Discharge time is known as circuit turn-off time.
110Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
C
Circuit turn-off time is given by
Where is load current.
t depends on load current, it must be designed
for the worst case condition which occur at the
maximum value of load current and mini
CC
L
L
V Ct
I
I
mum
value of capacitor voltage.
111Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Capacitor recharges back to the supply voltage (with plate ‘a’ positive).
• This time is called the recharging time and is given by
• The total time required for the capacitor to discharge and recharge is called the commutation time and it is given by
Sd
L
V Ct
I
r C dt t t
112Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• At the end of Mode-2 capacitor has recharged to VS and the free wheeling diode starts conducting.
113Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Mode-3 Operation
LOAD
C
L S
V S
+_
+
_
T 2V S
FW D
IL
IL
114Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• FWD starts conducting and the load current decays.
• The energy stored in source inductance LS is transferred to capacitor.
• Hence capacitor charges to a voltage higher than supply voltage, T2 naturally turns off.
115Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
The instantaneous capacitor voltage is
sin
Where
1
SC S L S
S
S
LV t V I t
C
L C
116Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Mode-4 Operation
LOAD
C
L S
V S
+_
+
_
D 1
LFW D
IL
V C
117Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Capacitor has been overcharged i.e. its voltage is above supply voltage.
• Capacitor starts discharging in reverse direction.
• Hence capacitor current becomes negative. • The capacitor discharges through LS, VS, FWD,
D1 and L. • When this current reduces to zero D1 will stop
conducting and the capacitor voltage will be same as the supply voltage
118Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Mode-5 Operation
• Both thyristors are off and the load current flows through the FWD.
• This mode will end once thyristor T1 is fired.
LOAD
IL
FW D
119Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
C a p a c i to r C u rr e n tI L
t
t
I p C u r r e n t t h r o u g h T 1
ic
0I p
iT1
0
I L
120Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
t
t
t
Vo l ta g e a c r o s s T 1
O u tp u t Vo l t a g e
C a p a c i to r Vo l t a g e
tctd
v T1
V c
0v o
V s c+ V
V s
v c
V c
- V c
121Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
Disadvantages• A starting circuit is required and the starting circuit
should be such that it triggers thyristor T2 first.
• Load voltage jumps to almost twice the supply voltage when the commutation is initiated.
• The discharging and charging time of commutation capacitor are dependent on the load current and this limits high frequency operation, especially at low load current.
122Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
• Chopper cannot be tested without connecting load.
• Thyristor T1 has to carry load current as well as resonant current resulting in increasing its peak current rating.